Considerable efforts have been made to discover new organic semiconductor materials that can be used in FET's to provide switching or logic elements in electronic components, many of which require significant mobilities well above 0.01 cm2/V.sec, and current on/off ratios (hereinafter referred to as “on/off ratios”) greater than 1000. Organic FET's (“OFET's”) having such properties can be used for electronic applications such as pixel drivers for displays and identification tags. However, most of the compounds exhibiting these desirable properties are “p-type” or “p-channel,” meaning that negative gate voltages, relative to the source voltage, are applied to induce positive charges (holes) in the channel region of the device.
As an alternative to p-type organic semiconductor materials, n-type organic semiconductor materials can be used in FET's where the terminology “n-type” or “n-channel” indicates that positive gate voltages, relative to the source voltage, are applied to induce negative charges in the channel region of the device.
Moreover, one important type of FET circuit, known as a complementary circuit, requires an n-type semiconductor material in addition to a p-type semiconductor material. Simple components such as inverters have been realized using complementary circuit architecture. Advantages of complementary circuits, relative to ordinary FET circuits, include lower power dissipation, longer lifetime, and better tolerance of noise. In such complementary circuits, it is often desirable to have the mobility and the on/off ratio of an n-channel device similar in magnitude to the mobility and the on/off ratio of a p-channel device. Hybrid complementary circuits using an organic p-type semiconductor and an inorganic n-type semiconductor are known, but for ease of fabrication, an organic n-channel semiconductor material would be desired in such circuits.
Only a limited number of organic materials have been developed for use as a semiconductor n-channel in OFET's. One such material, buckminsterfullerene C60, exhibits a mobility of 0.08 cm2/V.sec but it is considered unstable in air (Haddon et al. Appl. Phys. Let. 1995, 67, 121). Perfluorinated copper phthalocyanine has a mobility of 0.03 cm2/V.sec and is generally stable to air operation, but substrates must be heated to temperatures above 100° C. in order to maximize the mobility in this material (Bao et al. Am. Chem., Soc. 1998, 120, 207). Other n-channel semiconductors, including some based on a naphthalene framework, have also been reported, but with lower mobilities. One such naphthalene-based n-channel semiconductor material, tetracyanonaphthoquino-dimethane (TCNNQD), is capable of operation in air, but the material has displayed a low on/off ratio and is also difficult to prepare and purify.
Aromatic tetracarboxylic diimides, based on a naphthalene aromatic framework, have also been demonstrated to provide, as an n-type semiconductor. Thus, in naphthalene diimide-based OFET's, U.S. Pat. No. 6,387,727 (Katz et al.) teaches n-channel mobilities up to 0.16 cm2/V.sec. Comparable results were obtained with bottom contact devices, but a thiol underlayer had to be applied between the gold electrodes and the semiconductor as described. In the absence of the thiol underlayer, the mobility of the naphthalene diimide derivatives in U.S. Pat. No. 6,387,727 was found to be orders of magnitude lower in bottom-contact devices. This patent also discloses fused-ring tetracarboxylic diimide compounds, one example of which is N,N′-bis(4-trifluoromethyl benzyl)naphthalene diimide. The highest mobilities of 0.1 to 0.2 cm2/V.sec were reported for N,N′-dioctyl naphthalene diimide.
In a different study, using pulse-radiolysis time-resolved microwave conductivity measurements, relatively high mobilities have been measured in films of naphthalene diimides having linear alkyl side chains (Struijk et al., J. Am. Chem. Soc. Vol. 2000, 122, 11057).
U.S. Patent Application Publication 2002/0164835 (Dimitrakopoulos et al.) discloses n-channel semiconductor films made from perylene diimide compounds, as compared to naphthalene-based compounds, one example of which is N,N′-di(n-1H,1H-perfluorooctyl)perylene diimide. Substituents attached to the imide nitrogens in the diimide structure comprise alkyl chains, electron deficient alkyl groups, and electron deficient benzyl groups, and the chains preferably having a length of four to eighteen atoms. Devices based on materials having a perylene framework used as the organic semiconductor have low mobilities, for example 10−5 cm2/V.sec for perylene tetracarboxylic dianhydride (PTCDA) and 1.5×10−5 cm2/V.sec for N,N′-diphenyl perylene diimide (PTCDI-Ph) (Horowitz et al. Adv. Mater. 1996, 8, 242 and Ostrick et al. J. Appl. Phys. 1997, 81, 6804).
In perylene and naphthalene diimide based OFET's, many experimental studies have demonstrated that morphology of the thin film has strong impact on the device performances. Theoretical calculation and experimental characterization (particularly X-ray diffraction), have shown that the molecular packing in PDI is very sensitive to the side chains (Kazmaier et al. J. Am. Chem. Soc. 1994, 116, 9684). In perylene diimide based n-channel OFET devices, changing the side chain from n-pentyl to n-octyl increases the field effect mobility of from 0.055 cm2/V.sec to 1.3 cm2/V.sec, respectively (Chesterfield et al. J. Phys. Chem. B 2004, 108, 19281). Such sensitivity to the type of side-chain is a manifestation of an aggregation effect and it provides potentially an effective way to control and optimize the molecular packing for enhanced π-orbital overlap between neighboring molecules, a necessary for efficient carrier transport. U.S. Pat. No. 7,422,777 (Shukla et al.) discloses N,N′-dicycloalkyl-substituted naphthalene diimide compounds, which in thin films, exhibit optimum packing and exhibit n-channel mobility up to 6 cm2/V.sec in OFET's. U.S. Pat. No. 7,579,619 (Shukla et al.) discloses N,N′-di(arylalkyl) substituted naphthalene diimide compounds that exhibit high n-channel mobility up to 3 cm2/V.sec in top-contact OFET's.
A variety of naphthalene diimides have been made and tested for n-type semiconducting properties. In general, these materials, as an n-type semiconductor, have provided n-channel mobilities up to 6 cm2/V.sec using top-contact configured devices. However, besides charge mobility, improved stability and integrity of the semiconductor layer are important goals.
U.S. Patent Application Publication 2005/0176970 (Marks et al.) discloses improved n-channel semiconductor films made of mono- and diimide perylene and naphthalene compounds wherein the nitrogen and core are substituted with electron withdrawing groups. Substituents attached to the imide nitrogen's in the diimide structure can be selected from alkyl, cycloalkyl, substituted cycloalkyl, aryl, and substituted aryl groups. However, this publication fails to suggest any comparative advantage of using cycloalkyl groups on the imide nitrogen atoms. Accordingly, mobilities obtained from perylene diimides containing of N-octyl and N-cyclohexyl are virtually indistinguishable (see Example 10 of the publication). Furthermore, the highest mobilities reported in this reference are 0.2 cm2/V.sec and the reference fails to show experimental data with respect to naphthalene compounds and require that their core be cyano di-substituted.
Aromatic tetracarboxylic diimides, based on a naphthalene and perylene aromatic framework have been widely used as n-type semiconductor materials (Newman et al. Chem. Mater. 2004, 16, 4436-4451). Relatively low mobilities have been measured in films of naphthalene tetracarboxylic diimides having linear alkyl side chains using pulse-radiolysis time-resolved microwave conductivity measurements. See Struijk et al. “Liquid Crystalline Perylene Diimides: Architecture and Charge Carrier Mobilities” J. Am. Chem. Soc. Vol. 2000, 122, 11057. However, TFT's based on N,N′-dicyclo-substituted naphthalene diimide exhibit mobility up to 5 cm2/V.sec (Shukla et al. Chem. Mater. 2008, 20, 7486-7491). U.S. Pat. No. 6,387,727 (Katz et al.) discloses fused-ring tetracarboxylic diimide compounds, such as N,N′-bis(4-trifluoromethyl benzyl)naphthalene-1,4,5,8,-tetracarboxylic acid diimide. The highest mobilities reported in this patent is between 0.1 and 0.2 cm2/V.sec for N,N′-dioctyl naphthalene-1,4,5,8-tetracarboxylic acid diimide.
It is widely recognized that the morphology and microstructure of an organic thin film has a strong impact on the charge carrier mobility and OTFT device characteristics. In general, organic materials that form highly oriented polycrystalline thin films exhibit high charge carrier mobility. At the molecular level, it is the basic chemical structure of the molecule that controls intermolecular interactions that determines if a material will be crystalline or amorphous. The extent of π-stacking between the molecules determines whether the organic film will be highly crystalline or totally amorphous. Thus, to have well-defined thin film morphology, it is necessary to control materials on the molecular scale. This necessitates adapting the basic structure of semiconducting molecules in a way that results in an optimum crystalline packing arrangement.
It is known that diimide based semiconductors are very sensitive to the substitutions on the nitrogen atoms of the diimide rings. Such sensitivity to the side-chain is a manifestation of subtle changes in diimide aggregation in solid state and provides potentially an effective way to control and optimize the molecular packing for enhanced π-orbital overlap between neighboring molecules, a necessity for efficient carrier transport. Accordingly, U.S. Pat. No. 7,422,777 (Shukla et al.) discloses N,N′-dicycloalkyl-substituted naphthalene diimide compounds, which in thin films, exhibit optimum packing and exhibit n-channel mobility up to 6 cm2/V.sec in OFET's. In another example, U.S. Pat. No. 7,579,619 (Shukla et al.) discloses N,N′-di(arylalkyl) substituted naphthalene diimide compounds that exhibit high n-channel mobility up to 3 cm2/V.sec in top-contact OFET's. These materials consistently exhibit higher mobility compared to a naphthalene tetracarboxylic diimide having phenyl substituents.
U.S. Patent Application Publications 2008/0135833 (Shukla et al.) and 2009/0256137 (Shukla et al.) describe n-type semiconductor materials for thin film transistors that include configurationally controlled N,N′-dicycloalkyl-substituted naphthalene 1,4,5,8-bis-carboximide compounds or N,N′-1,4,5,8-naphthalenetetracarboxylic acid imides having a fluorinated substituent, respectively. In these cycloalkyl-substituted naphthalene diimide derivatives, the effect of the alkyl group configuration in the cycloalkyl ring affects the aggregation, and hence the carrier mobility, in solid state.
Recently, dicyanated arylene diimide semiconductors based on perylene and naphthalene diimide cores have been developed that are solution processable and show environmental stability (Adv. Funct. Mater. 2008, 18, 1329-1339). The latter characteristics arise from cyano group addition to the core, which increases solubility by decreasing molecular planarity and stabilizes charge carriers by lowering the energies of the lowest unoccupied molecular orbital's associated with electron transport. While high temperature vapor deposited devices using these materials show good mobilities (ca. 0.1-0.5 cm2/V.sec; Jones et al. Adv. Funct. Mater. 2008, 18, 1329-1339), solution coated device usually give lower mobility and exhibit low Ion/Ioff ratio.
As is clear from the foregoing discussion, the development of new semiconducting materials, both p-type and n-type, continues to be an enormous topic of interest and unpredictable as to the semiconductive properties of various compounds. Among n-type diimide based materials, the highest charge carrier mobility (ca. 5.0 cm2/V.sec) in thin film transistors has been observed with N,N′-dicyclohexyl naphthalene diimide. However, the poor solubility of this material limits its practical application potential. Although, as discussed above, dicyanated arylene diimide semiconductors based on perylene and naphthalene diimide cores are solution processable and show environmental stability their carrier mobility is low. To attain solubility extensive molecular modification have to be made which usually lowers the crystallinity of the material (for example see et al. Adv. Funct. Mater. 2008, 18, 1329-1339) that usually results in lower mobility in OTFT devices.
Efforts continue to improve performance of n-type organic semiconductor materials in OTFT's and technology for their manufacture and use. Specifically there continues to be research efforts to find new materials and processes that are useful in n-type semiconducting materials which compounds do not require significant structural modification to achieve processability and optimum crystalline packing.
Amic acids are usually more soluble than aromatic anhydrides they are derived from. One attractive way of obtaining solution-processed thin films of diimide based semiconductors is to solution coat an amic acid and then by thermal dehydration reaction, convert it to the corresponding diimide.
The dehydration of amic acids, derived from the reaction of cyclic anhydrides with primary amines, to yield imides is a general method for the preparation of this important class of heterocyclic compounds and is of major commercial significance in the conversion of polyamic acids to polyimides (Kreuz, Endrey, Gay, and Sroog, J. Polym. Sci., Part A, 4, 2607 (1966), and references contained therein.). As polyimides derived from phthalamic acids possess many desirable attributes, this class have materials have found applications in many technologies ranging from dielectrics in microelectronics to high temperature adhesives to membranes (for example see Mittal, Polyimides and Other High Temperature Polymers: Synthesis, Characterization and Applications vol. 1 to 5). Most of the detailed studies have concentrated on preparation of polyphthalamic acids and their conversion to polyimides in solid films (for example, see Kim et al. in Polymer 40, 1999, pp 2263-2270, and references cited therein). In contrast, little is known about the dehydration reactions of amic acids derived from anthracene, naphthalene, and perylene anhydrides or anthracene, naphthalene, and perylene tetracarboxylic acid dianhydrides. Fabienne et al have recently reported mechanistic studies of polycondensation reactions of naphthalene anhydride leading to naphthalimide polymers (Piroux, Mercier, and Picq, High Performance Polymers (2009), 21(5), 624-632).
Genies et al. have reported synthesis of soluble sulfonated naphthalenic polyimides, derived from naphthalene dianhydride, as materials for proton exchange membranes (Genies et al. Polymer 42 (2001) 359-373).
Copending and commonly assigned U.S. Ser. No. 12/770,803 (filed Apr. 30, 2010 by Shukla, Meyer, and Ahearn) describes novel aromatic amic acids and amic esters that can be thermally converted to corresponding arylene diimides that are formed into semiconductive layers for various articles and devices. These compounds are advantageous in that the semiconductive layers can be formed in situ while the precursor compounds are readily coated from organic solvents.
Salts of poly(amic acids) have also been shown to undergo thermal imidization reaction to generate polyimides. Facinelli et al. have prepared thermoplastic polyimides via poly(amic acid) salt precursors (see Facinelli et al. Macromolecules 1996, 29, 7342-7350). These poly(amic acid) salts were prepared in heterogeneous reactions of the poly(amic acid)s using quaternary ammonium bases or triethylamine dissolved in methanol or water to yield soluble salts which were then melt imidized in air at 250° C. or 300° C. for 30 minutes. Ding et al. have prepared polyimide based membranes from poly(amic acid) salts (see Ding et al. Macromolecules 2002, 35, 905-911). This publication shows that poly(amic acid) tertiary amine salts can be quantitatively imidized at a lower temperature than the poly(amic acid) or poly(amic acid) quaternary amine salts of identical backbone structure. Xu et al. have synthesized polyimides from a diamine-acid salt and a dianhydride in the presence of excess triethylamine, thereby avoiding the use of air-sensitive aromatic diamine compounds as monomers (Xu et al., Macromol. Rapid Commun. 2000, 21, 481-484). Yang et al. have also prepared and characterized poly(amic acid) salts of pyromellitic dianhydride (Yang et al. Macromolecular Research, 2004, Vol. 12, No. 3, pp 263-268). Polyimide multilayer thin films prepared from poly(amic acid) and poly(amic acid) ammonium salt are described in Macromolecular Research, 2008, Vol. 16, No. 8, pp 725-733. WO 95/04305 (Flattery et al.) discloses a photosensitive composition of a fluorinated poly(amic acid) aminoacrylate salt.
WO 92/00283 (Goze et al.) discloses the use of N,N′-disubstituted amic acid ammonium salts, their use as surfactants, emulsifiers, suspending agents, and conditioning agents in shampoos. This publication does not teach thermal imidization reaction of such salts. It also fails to disclose amic acids salts of naphthalene or perylene tetracarboxylic acids.
Kim et al. J. Phys. Org. Chem. 2008, 21 731-737 describes the formation of amic acid salts in the hydrolysis of certain aliphatic naphthalene diimides. However, the publication does not isolate these salts or teach their thermal imidization reaction. EP 0 805 154A1 (Iwasawa et al.) discloses certain N,N-disubstituted amic acid derivatives as in-vivo inhibitors of protein-farnesyl transferase (PFT).
The use of such amic acid and amic ester precursor compounds have a number of advantages, as described in the U.S. Ser. No. 12/770,803 described above, but there is a need to provide semiconductive layers at lower temperatures, or even at room temperature, to improve manufacturing processes.